US 7015115 B1
According to one embodiment, a structure comprises a substrate and a field oxide region, where the field oxide region has a top surface, and where the top surface of the field oxide region comprises substantially no cavities caused by lateral etching. The structure further comprises a trench situated in the substrate, where the trench has a first sidewall and a second sidewall in the substrate, and where the trench is situated directly underneath the field oxide region. According to this embodiment, the trench is used as a deep trench isolation region in the substrate and is typically filled with polysilicon. A thermally grown oxide liner is situated on the first and the second sidewalls of the trench, where the oxide liner is formed after removal of a hard mask. The hard mask may be densified TEOS oxide or HDP oxide and may be removed in an anisotropic dry etch process.
1. A method for forming a deep trench isolation region in a substrate, said method comprising steps of:
forming a layer of nitride over said substrate and over a first field oxide region in said substrate;
forming a hard mask over said layer of nitride;
forming a trench in said first field oxide region and said substrate, said trench defining first and second sidewall in said substrate, said forming said trench causing first and second field oxide portions to be formed adjacent to said first and second sidewalls of said trench, respectively;
removing said hard mask after said forming said trench;
thermally growing an oxide liner on said first and second sidewalls of said trench after said removing said hard mask;
conformally depositing a layer of polysilicon on said layer of nitride and in said trench, said layer of polysilicon filling said trench;
forming a continuous second field oxide region directly above said trench by growing oxide inside said trench to merge said first and second field oxide portions;
wherein substantially no lateral etching occurs in said first and second field oxide portions prior to said thermally growing said oxide liner;
and wherein a substantially flat topography is achieved over said deep trench isolation region.
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1. Field of the Invention
The present invention relates to the field of fabrication of semiconductor devices. More specifically, the invention relates to forming isolation regions in semiconductor substrates.
2. Background Art
In a Bipolar Complementary-Metal-Oxide-Semiconductor (“BiCMOS”) process, deep trench isolation regions are typically formed to provide isolation between adjacent active regions of a semiconductor substrate. A deep trench isolation region may electrically isolate, for example, a bipolar transistor, such as a silicon-germanium heterojunction bipolar transistor, from an adjacent CMOS transistor, such as a PFET, fabricated on the same semiconductor substrate. Deep trench isolation regions may be formed on a substrate after formation of, for example, field oxide isolation regions, a buried layer, and an epitaxial layer of silicon.
In a typical conventional deep trench isolation process flow, a layer of silicon nitride (“nitride”) is deposited on a silicon substrate at a thickness of approximately 1500 Angstroms, for example. A hard mask having an approximate thickness of 4500 Angstroms and comprising densified tetraethylorthosilicate (“TEOS”) oxide or high-density plasma (“HDP”) oxide is then formed over the nitride layer. A photoresist mask is formed and patterned over the hard mask, and a trench is etched to a depth just below a field oxide region. After removal of the photoresist mask, the trench is further etched into the silicon substrate to a depth of between approximately 7.0 and 10.0 microns. During trench etching, the hard mask is also etched, which results in a reduction in hard mask thickness by approximately one half. After performance of cleaning and other preparatory steps as known in the art, a densified TEOS oxide liner is formed on the sidewalls of the trench and on the surface of the silicon substrate. The TEOS oxide liner may have a thickness of approximately 1000.0 Angstroms or greater.
Next, in the conventional deep trench isolation process discussed above, a conformal layer of polycrystalline silicon (also referred to as polysilicon) is deposited over the silicon substrate and trench. The layer of polysilicon is then recess etched in the trench to a depth of approximately 1500.0 Angstroms below the interface of the field oxide region and the nitride layer. The densified TEOS oxide liner situated on the sidewalls of the deep trench and on the silicon substrate surface is then removed in a wet etch process utilizing a buffered oxide etchant (“BOE”). However, since the polysilicon is recess etched to a depth of approximately 1500.0 Angstroms below the nitride layer, the BOE wet etch causes severe lateral etching of portions of the field oxide region situated on the sidewalls of the trench.
In an attempt to reduce the severe field oxide undercutting discussed above, semiconductor manufacturers have utilized a dry etch in place of the BOE wet etch. However, the dry etch approach causes formation of a non-uniform “mini-trench” or cavity on either side of the polysilicon-filled trench. As a result, removal of the densified TEOS oxide liner utilizing either the BOE wet etch or the dry etch approach causes undesirable deep mini-trenches or cavities to be formed in the field oxide.
Thus, there is a need in the art for effective deep trench isolation in a semiconductor substrate.
The present invention is directed to method for forming deep trench isolation and related structure. The present invention addresses and resolves the need in the art for effective deep trench isolation in a semiconductor substrate.
According to one exemplary embodiment, a structure comprises a substrate. The structure further comprises a field oxide region situated in the substrate, where the field oxide region has a top surface, and where the top surface of the field oxide region comprises substantially no cavities caused by lateral etching. The field oxide region may have a thickness of, for example, approximately 3000.0 Angstroms. The structure further comprises a trench situated in the substrate, where the trench has a first sidewall and a second sidewall in the substrate, and where the trench is situated directly underneath the field oxide region. According to this exemplary embodiment, the trench is used as a deep trench isolation region in the substrate and is typically filled with polysilicon. The trench may have a depth of, for example, between approximately 3.0 microns and approximately 7.0 microns and may have a width of, for example, approximately 1.0 micron.
According to this exemplary embodiment, the structure further comprises a thermally grown oxide liner situated on the first and the second sidewalls of the trench, where the oxide liner is formed after removal of a hard mask. The oxide line may have a thickness of, for example, approximately 1000.0 Angstroms. The hard mask may be densified TEOS oxide or HDP oxide and may be removed in an anisotropic dry etch process, for example. According to one embodiment, the invention is a method for achieving the above-described structure. Other features and advantages of the present invention will become more readily apparent to those of ordinary skill in the art after reviewing the following detailed description and accompanying drawings.
The present invention is directed to method for forming deep trench isolation and related structure. The following description contains specific information pertaining to the implementation of the present invention. One skilled in the art will recognize that the present invention may be implemented in a manner different from that specifically discussed in the present application. Moreover, some of the specific details of the invention are not discussed in order to not obscure the invention. The specific details not described in the present application are within the knowledge of a person of ordinary skill in the art.
The drawings in the present application and their accompanying detailed description are directed to merely example embodiments of the invention. To maintain brevity, other embodiments of the invention which use the principles of the present invention are not specifically described in the present application and are not specifically illustrated by the present drawings.
The present invention involves a method of forming deep trench isolation that causes substantially no lateral etching of field oxide. As will be discussed in detail below, by causing substantially no lateral etching of field oxide, the present invention advantageously achieves a final topography that is substantially flat over a deep trench isolation region. The innovative method of the present invention can be applied in, for example, BiCMOS applications to achieve effective formation of deep trench isolation regions.
Moreover, structures 250 through 264 in
Referring now to
Continuing with step 150 in
Referring to step 152 in
In a second etch step, the photoresist mask can be removed from hard mask 216 and polymer residue formed as a result of the first etch step discussed above can be removed from sidewalls 222 and 224 of trench 220 in a wet strip utilizing an appropriate etchant as known in the art. The width of trench 220 is determined by the CD of the trench opening patterned in the photoresist mask discussed above, and can be, for example, approximately 1.0 micron. The result of step 152 of flowchart 100 is illustrated by structure 252 in
Referring to step 154 in
Referring to step 156 in
Referring to step 158 in
In the channel stop implant performed in step 158, a dopant such as boron, for example, can be implanted in bottom surface 226 of trench 220 to prevent current leakage. A wet strip can be performed to eliminate carbon and metal contamination resulting from the channel stop implant. Also, prior to thermally growing oxide liner 228, a thirty-second HF preclean can be performed to appropriately clean sidewalls 222 and 224 and bottom surface 226 of trench 220.
Next, oxide liner 228 is formed by thermally growing oxide on sidewalls 222 and 224 and bottom surface 226 of trench 220. The layer of thermal oxide can be densified by utilizing, for example, a temperature of approximately 750° C. and an inert gas such as nitrogen to complete formation of oxide liner 228. By way of example, oxide liner 228 can have a thickness of approximately 1000.0 Angstroms.
Since oxide liner 228 comprises thermally grown oxide, oxide liner 228 is not formed on nitride layer 214 situated on top surface 218 of silicon substrate 202. By not forming oxide liner 228 on nitride layer 214, a buffered oxide echant (“BOE”) wet strip does not need to be performed in a subsequent step to remove oxide liner 228. As a result, the present invention advantageously eliminates field oxide lateral etching or undercutting caused by a BOE wet strip. By eliminating field oxide lateral etching, the present invention advantageously achieves a substantially flat final topography over a subsequently formed deep trench isolation region. The result of step 158 of flowchart 100 is illustrated by structure 258 in
Referring to step 160 in
Referring to step 162 in
As discussed above, hard mask 216 is removed at step 156. By removing hard mask 216 at step 156, the present invention's deep trench isolation process advantageously avoids lateral etching in field oxide portions 209 and 211 that can occur as a result of removing hard mask 216. As a result, the present invention advantageously achieves a field oxide region, i.e. field oxide region 240, having substantially no cavities on its top surface, i.e. top surface 236.
In contrast, in a conventional deep trench isolation process, the hard mask is generally removed after the polysilicon recess etch, which occurs at step 160. In addition, in the conventional process, the oxide liner is removed with the hard mask, which further increases the thickness of the material that must be removed in an etching process. Furthermore, since, in the conventional process, the hard mask has a non-uniform thickness as a result of hard mask etching that occurs during trench etching, the material to be removed has a non-uniform thickness as well as an overall increased thickness. As a result, in the conventional process, a wet strip, such as a BOE wet strip, cannot be used to remove the hard mask and oxide liner without causing severe field oxide undercutting. Thus, in the conventional process, a dry etch process is used to remove the hard mask and oxide liner. However, the dry etch process results in formation of undesirable deep cavities in the top surface of field oxide situated above the deep trench isolation region. The result of step 162 of flowchart 100 is illustrated by structure 262 in
Referring to step 164 in
It is appreciated by the above detailed description that the invention provides a method for effectively forming a deep trench isolation region. From the above description of the invention it is manifest that various techniques can be used for implementing the concepts of the present invention without departing from its scope. Moreover, while the invention has been described with specific reference to certain embodiments, a person of ordinary skill in the art would appreciate that changes can be made in form and detail without departing from the spirit and the scope of the invention. Thus, the described embodiments are to be considered in all respects as illustrative and not restrictive. It should also be understood that the invention is not limited to the particular embodiments described herein but is capable of many rearrangements, modifications, and substitutions without departing from the scope of the invention.
Thus, method for forming deep trench isolation and related structure have been described.